1. Field
This relates to circuits for fluorescent lamps, including ballast circuits having sensing and/or control circuits that operate in part in a manner relating to incipient failure of the lamp and which produce a limited number of multiple re-strikes of a lamp.
2. Related Art
Power supply circuits such as ballasts for fluorescent lamps may include integrated circuits. These ballasts are electronic ballasts, and they are widely used to power lighting circuits, including conventional fluorescent lamps, compact fluorescent lamps, and other fluorescent lighting components. Electronic ballasts are capable of performing a number of functions, which may include pre-heating of the lamp filaments, driving inverter circuits for providing AC power to the lamp, and various shutdown functions. Some proprietary integrated circuits may also include end-of-lamp life shutdown circuits, automatic ballast reset upon lamp replacement and control of the number of re-start attempts before shutting off the ballast.
Some ballasts may be designed with commercially available driver chips. Additional circuits are provided to give end-of-lamp life detection, and other circuits may provide other functions. End-of-lamp life detection may sense slightly increased voltage (for example about 10%) across the lamp for a prolonged time, which may indicate an aging lamp, or may sense a very high voltage (for example about four times higher) across the lamp for a brief period (for example about 50 microseconds), which may indicate a missing lamp or a degassed lamp. Increasing voltage across the load will typically cause a ballast to increase the power delivered to the load. Sustained higher power delivery may cause the ballast to over heat or possibly fail, or the lamp to shatter.